Please refer to FIG. 1, which shows a typical GFSK receiver structure. The antenna 100 receives a GFSK signal 110 propagating through channel, then the RF circuit 120 removes the carrier from the received GFSK signal 110 and obtains a baseband analog complex signal z(t) 130. The purpose of ADC (analog-to-digital converter) 140 is to sample the analog baseband complex signal 130 to a digital baseband complex signal zk,j 150, and the baseband circuit 160 demodulates and processes the digital baseband complex signal 150 and obtains the original binary sequence b(k) 170.
The invention is related to the digital modem system using GFSK scheme to transmit the signal in the baseband. The digital baseband complex signals are demodulated using differential detection. In other word, the decision of a bit is based on the phase difference between the current and its previous received signals. In addition, to demodulate a signal correctly also requires certain mechanism for synchronization. The synchronization tasks include carrier frequency, phase and symbol timing. Generally, a communication system will provide extra information (so-called preamble or training sequence) to aid the receiver to accomplish these tasks. This present invention develops algorithms to perform differential detection of GFSK signal using decision feedback and preamble (training sequence) for frequency, clock and frame synchronization.
GFSK, which employs Gaussian filter for pulse shaping, is an attractive modulation scheme due to its compact spectrum. However, the Inter-Symbol-Interference (ISI) introduced by the Gaussian filter also degrades the bit error rate (BER) performance. Various receiver structures were proposed to improve the BER performance of GFSK owing to the ISI resulted from Gaussian filter. ABBAS et al (reference b.1) proposed a method using differential detection with decision feedback to overcome the ISI issue. In their original paper, they only dealt with GMSK modulation and assumed that clock and frequency have been synchronized perfectly.
As to the synchronization issue, Mehian et al (reference b.2) proposed a method to estimate the symbol timing and frequency offset without training sequence. In their original paper, they only dealt with GMSK modulation and used conventional differential detection.
This present invention modifies, combines, and extends their works from GMSK to GFSK. For a given pre-known preamble, this invention estimates the frequency offset using the preamble as prior information and estimate the symbol timing using the estimated frequency offset. Once the estimated frequency offset and symbol timing are obtained, this information is used to do differential detection incorporating with decision feedback.